Nickel-cadmium batteries generally consist of a wound anode interleaved with a wound cathode, the wound anode and cathode being spaced apart at regular intervals in an electrolyte. The interval between the anode and cathode may be as small as 0.05 mm. Although it is desirable to place the cathode and anode close together to increase the load capacity of the battery, the electrodes must not touch to avoid producing a short circuit. To accomplish this end, separators made of suitable material are arranged between the anode and cathode to keep them apart. The separator material must be inert to the electrolyte and to the reactions occurring at the surfaces of the electrodes.
In addition, the separator material should be sufficiently elastic to conform to the shape of the electrode surfaces. Also the separator material should be sufficiently porous to allow unimpeded migration of ions between the electrodes, yet be able to filter out solid particles which separate from the electrodes and attempt to pass through the separator. The separator material further must be wettable by the liquid electrolyte to prevent the establishment of dry areas on the separator fabric. Finally, the separator should have the capacity to adsorb and store the liquid electrolyte.
Separator material made from woven fabric is disadvantageous because fabric stores insufficient quantities of the liquid electrolyte. Furthermore, because pores formed between the warp and weft of the fabric are large, solid particles which dislodge from the electrodes can pass through the fabric. Such particles accrete until a bridge is formed between an anode and cathode, giving rise to a short circuit in the battery.
It is known in the prior art that the foregoing disadvantages can be overcome by providing a battery separator material made from nonwoven polypropylene fabric.
U.S. Pat. No. 3,615,865 to Wetherell discloses a battery separator comprising a nonwoven mat of polypropylene fibers bonded with polyacrylic acid. In lieu of polypropylene fibers, polyethylene or polyamide fibers may be used.
U.S. Pat. No. 4,205,122 to Miura et al. discloses a method for manufacturing a battery separator material by subjecting an aqueous dispersion of olefinic resin fibers to a sheet-forming operation; drying the resulting wet nonwoven mat; and heat-treating the dried mat to form a self-supporting nonwoven mat. The drying and heat treatment of the nonwoven mat can be performed by passing it through a hot air dryer or "by means of dryers used in conventional papermaking machines, such as a Yankee dryer". After heat treatment, the mat is preferably calendared to increase the surface smoothness.
U.S. Pat. No. 4,216,281 to O'Rell et al. discloses a battery separator comprising 30-70% polyolefin synthetic pulp, 15-65% siliceous filler and 1-35% by weight of long fibers made of polyester or glass. Cellulose may be included in an amount up to 10%. The battery separator material is formed using standard papermaking equipment. The papermaking equipment disclosed in the O'Rell '281 patent comprises a pulper, a chest, a head box and a rotoformer drum which rotates in the head box to pick up slurry and form a web. The web is removed from the rotoformer drum and passed over a felt. The web is pressed by calendars. The calendared web is fed to an oven and then onto a series of heated cans. The cans feed to a windup station. In Example 1, the steam cans were operated at surface temperatures of about 270.degree. F. The steam cans both dried the web and increased fiber bonding.
U.S. Pat. No. 4,430,398 to Kujas discloses a battery separator comprising a nonwoven mat of polypropylene fibers. In the Example, 4-denier polypropylene fibers were used.
U.S. Pat. No. 4,987,024 to Greenberg et al. discloses a battery separator comprising a fibrous web having 90% bicomponent polypropylene/polyethylene 0.9-denier, 38-mm-long fiber and 10% polypropylene 1.5-denier, 38-mm-long fiber (see Example II). The nonwoven web is produced on web-forming equipment "such as cards, air formed, melt blowing, continuous filament or even wet-lay equipment". In accordance with the method of Greenberg, the web is passed through the nip of two steel rolls that are heated and have pressure applied to them. The heated rolls sear the top and bottom surfaces of the web to produce film-like surfaces.
U.S. Pat. No. 5,180,647 to Rowland et al. discloses a battery separator comprising a fibrous mat which is filled with a mixture of very fine mineral powder (e.g., silicon material) and a binder resin (e.g., latex). Rowland discloses that the mat can be a wet-laid polypropylene nonwoven material. However, the only examples given in Rowland are spunbonded webs made from polyester fibers.
U.S. Pat. No. 5,298,348 to Kung discloses a battery separator for nickel/metal hydride batteries. The battery separator is formed from 10-20 wt. % polyolefin pulp, one or more non-water-swellable fibers and at least one water-swellable polyvinyl alcohol fiber. The separator is impregnated with 1 wt. % or less acrylic resin binder. The length of the polyolefin pulp is about 1 to 4 mm. The non-water-swellable fibers may be polyester, polyacrylic, polyamide, polyolefin, bicomponent fibers and mixtures thereof. The preferred bicomponent fiber is a fiber which has a polyethylene sheath and a polypropylene core. The sheet is formed on conventional papermaking machinery, such as a rotoformer or Fourdrinier paper machine. The web is dried in an oven and/or by one or more drying cans. The dried web may be calendared.
U.S. Pat. No. 5,436,094 to Horimoto et al. discloses a bulky synthetic pulp sheet useful as a separator for sealed lead batteries. The pulp sheet contains 5-95 wt. % of a synthetic pulp and 5-50 wt. % of a polymer binder. The sheet is made by subjecting a mixture of synthetic pulp and fibrous binder to wet-laid sheet-making followed by a heat treatment. The pulp can consist of polyethylene, polypropylene, polyester, nylon or other polymers. The binder may take the form of synthetic pulps, synthetic fibers, sheath-core type composite fibers, resin powders and emulsions. The type of binder selected is dependent on which kind of synthetic pulp is used as the chief material.